1. Field of the Invention
The present invention relates to a process for producing polyphenylene sulfide films. More particularly, the present invention relates to a process for producing non-oriented polyphenylene sulfide films which have little possibility of suffering from breakage due to so-called edge fracture or other causes when the non-oriented polyphenylene sulfied films are biaxially stretched.
2. Description of the Background
Polyethylene terephthalate films have been widely used as a general-purpose industrial material in recent years. Lately, however, a film having higher heat-resistance than polyethylene terephthalate films is required for meeting increasing demands for saving energy, reduction in size and weight of equipment and enhancement of reliability. Attention has been focused recently on poly-p-phenylene sulfide films as a film that can satisfy these purposes. Poly-p-phenylene sulfide has excellent heat-resistance inherently, but its crystallization rate is faster than that polyethylene terephthalate, so that it is difficult to obtain a film having excellent thermal and mechanical properties, dimensional stability, flatness and uniformity in thickness by merely subjecting a poly-p-phenylene sulfide film made by an ordinary film-forming method to the biaxial stretching and heat-setting treatments. That is, when a non-oriented film is made from poly-p-phenylene sulfide by a method commonly used for forming polyethylene terephthalate films, the produced film is crystallized at the edge because of its high crystallization rate, so that when it is stretched in the machine direction, the edge of the film may not be duly stretched and tends to suffer from edge fracture or other trouble, thereby causing breakage of the film. As a solution to this problem, a method has been proposed in which the crystallized edge alone of the film is cut out with a knife or laser beams. According to this method, however, in the case where the cutout is made with a knife, the knife becomes increasingly blunt with time to make the film liable to be notched, thereby causing frequent break of the films when stretched in the machine direction and finally making it impossible to stretch film. Also, in the case where the cutout is made with laser beams, heat is generated to cause crystallization at the cutting part, and in this case, too, the stretching of the film in the machine direction becomes impossible. Therefore, for producing an oriented polyphenylene sulfide film on an industrial scale, it is essential that the stretching of the film can be accomplised in a stable and steady way, and unless this is made possible, it is difficult to attain a reduction of cost and polyphenylene sulfide film is unsuited for industrial use.
As a result of assiduous studies for solving these problems, it has been found by the present inventors that the problems can be solved by aptly combining the electrostatic adhesion method and the liquid application adhering method, and based on this finding, the present invention has been attained.